1. Field of the Invention
The present invention generally relates to an optical printing head used in a digital writing optical system in a solid scanning writing system, and to an image forming apparatus using the optical printing head, and, can be applied to a digital copier, a printer, a digital facsimile machine and so forth.
2. Description of the Related Art
Recently, as digital outputting apparatuses such as a digital copier, a printer, a digital facsimile machine and so forth are miniaturized, miniaturization of digital writing devices used therein is demanded.
Currently, digital writing systems can be roughly classified into two types.
One thereof is an optical scanning system in which a light flux emitted from a light source such as a semiconductor laser is deflected by an optical deflector, and is used for forming a beam spot by an optical scanning and imaging lens.
The other one is a solid scanning system in which light fluxes emitted from light-emitting component array light source such as an LED array are used for forming beam spots by an imaging component array.
In the optical scanning system, the length of light path is large because an optical deflector deflects a light flux.
In contrast to this, in the solid scanning system, it is possible to shorten the length of light path remarkably. Accordingly, it is possible to miniaturize the entirety of the apparatus, and, also, this system does not need a mechanical driving component such as an optical deflector.
As unity-magnification imaging components used in the solid scanning system, a roof prism lens array and a roof mirror lens array have been proposed.
The roof prism lens array is an imaging component array in which a plurality of roof prism lenses, each having an entrance surface, an exit surface and a prism surface formed integrally therein, are arranged.
The roof mirror lens array is an imaging component array including a lens array in which a plurality of lenses each having an entrance surface and an exit surface are arranged and a roof mirror array in which a plurality of roof mirrors each having two reflective surfaces at right angles to each other are arranged.
In the related art, an imaging component array such as the above-mentioned roof prism lens array or roof mirror lens array is set so that the focal length thereof has a unity-magnification relationship with respect to a configuration relationship between a light-emitting component array, the imaging component array and an image carrying body surface, and, is called a unity-magnification imaging component array forming an erecting system in an arranging direction.
xe2x80x98The unity-magnification relationshipxe2x80x99 is a relationship such that the imaging magnification (lateral magnification) in the optical system including the position of the object point at which the light-emitting component array is disposed, the imaging (image-forming) component array and the position of the image point at which the image carrying body surface is disposed is 1.
However, it has been confirmed that, in an imaging component array in which an entrance surface and an exit surface of the above-mentioned roof prism lens array or the entrance surface and exit surface of the lens array of the above-mentioned roof mirror lens array are spherical surfaces, when setting is made such that the focal length of the imaging component array has a unity-magnification relationship with respect to a configuration relationship between a light-emitting component array, the imaging component array and an image carrying body surface, the defocus position (position of the beam waist) at which the diameter of the beam spot becomes minimum in the depth curve of the diameter of the beam spot in the direction perpendicular to the arranging direction of each array is away from the image carrying body surface.
Further, in order to enlarge an allowance of an assembling error at a time an optical printing head is held in an image forming apparatus and a manufacturing error of an imaging component array, it is preferable to enlarge a defocus range in which the diameter of beam spot can be allowed for the practical use, that is, a so-called allowance of focal depth.
The present invention has been devised in order to solve the above-mentioned problems, and an object of the present invention is to provide an optical printing head and an image forming apparatus employing the optical printing head by which it is possible to make the position of the beam waist in the direction perpendicular to the array arranging direction be on or in the proximity of the image carrying body surface, and, also, to enlarge the allowance of focal depth.
An optical printing head according to the present invention comprises:
a light-emitting component array having a plurality of light-emitting components arranged therein; and
an imaging component array having a plurality of imaging components arranged therein each for imaging from a light flux from a respective light-emitting component of the light-emitting component array to a beam spot on an image carrying body surface,
wherein, in a configuration relationship between the light-emitting component array, imaging component array and image carrying body surface, a focal length of the imaging components is slightly larger than the focal length of the imaging components obtained in a condition in which a unity-magnification relationship is satisfied.
Thereby, it is possible to locate the position of the beam waist in the arranging perpendicular direction on or in the proximity of the image carrying body surface, and, also, to enlarge the allowance of focal depth.
It is preferable that
1.005xe2x89xa6f/fxe2x80x2xe2x89xa61.025
where:
f denotes the focal length of the imaging components; and
fxe2x80x2 denotes the focal length of the imaging components obtained in the condition in which the unity-magnification relationship is satisfied.
Thereby, it is possible to locate the position of the beam waist in the arranging perpendicular direction on or in the proximity of the image carrying body surface, and, also, to enlarge the allowance of focal depth, more effectively.
It is preferable that
the focal length is made slightly longer as a result of radiuses of curvatures of surfaces having power of the imaging components being changed from radiuses of curvatures of surfaces having power of the imaging components obtained in the condition in which the unity-magnification relationship is satisfied.
Thereby, it is possible to avoid change of the optical layout and to eliminate influence on manufacturing method and costs of the imaging components.
It is preferable that an arranging pitch of the imaging component array is equal to or shorter than 1 mm.
Thereby, it is possible to prevent vertical strips in a resulting image from being remarkable, and to reduce the aberration of the imaging components to a smaller value.
It is preferable that the imaging component array comprises a roof prism lens array in which entrance surfaces are equal to exit surfaces in radius of curvature in absolute value.
Thereby, it is possible to easily reduce the component costs.
It is preferable that
1.005xe2x89xa6R/Rxe2x80x2xe2x89xa6R 1.03
where:
R denotes the absolute value of the radiuses of curvatures of the entrance surfaces and exit surfaces of the imaging component array; and
Rxe2x80x2 denotes an absolute value of radiuses of curvatures of entrance surfaces and exit surfaces of the imaging component array obtained in the condition in which the unity-magnification relationship is satisfied.
Thereby, it is possible to locate the position of the beam waist in the arranging perpendicular direction on or in the proximity of the image carrying body surface, and, also, to enlarge the allowance of focal depth, more effectively.
It is preferable that a ratio of an aperture diameter in an arranging perpendicular direction to an aperture diameter in an arranging direction is larger than 1.
Thereby, it is possible to locate the position the beam waist in the arranging perpendicular direction on or in the proximity of the image carrying body surface more effectively, and, also, to provide a brighter imaging optical system.
An image forming apparatus according to the present invention employs any of the above-mentioned optical printing heads as an exposure unit.
By using this image forming apparatus, it is possible to obtain a stable diameter of beam spot, and, to obtain a satisfactory image in which vertical stripes are not likely to be remarkable.
Other objects and further features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.